Liquid crystal materials are materials which occupy an intermediate state between crystalline solid materials and isotropic liquid materials. Liquid crystal materials, while exhibiting an orientational order, do not typically exhibit a positional order. The unique properties of liquid crystal materials have enabled their use in a variety of display applications. Among the useful properties of liquid crystal materials in display applications are the reflection and refraction of light by the liquid crystal (LC) and the ability of the user to influence these properties. These properties are governed by the orientation of the molecules which comprise the liquid crystal. The orientation of individual molecules often determines the behavior of layers and phases of these molecules.
The lack of mirror symmetry of individual molecules is described as the chirality or “handedness” of the molecule. Many liquid crystal phases are chiral due to the introduction of chirality of the same sign at the molecular level. Examples of these types of chiral liquid crystal phases include cholesteric, blue, Twist Grain Boundary (TGB) and smectic C* phases. Due to the long-range orientation order of liquid crystalline phases, and the chirality of the molecules, a spontaneous twist occurs in a micrometer range. The chirality transfers from a molecular to mesoscopic range, and the phase becomes chiral.
Two molecules that are identical in composition yet are mirror images of each other are described as having opposite chirality. This is generally expressed as the molecules being left-handed or right-handed depending on their particular orientation. Liquid crystal molecules having the same chemical formula but opposite chirality will behave in optically similar, but oppositely directed ways.
Scattering type devices are very well known in liquid crystal displays. Two known types are polymer dispersed liquid crystals (PDLC), and polymer network containing liquid crystals (PNLC). Liquid crystal polymer dispersions form a broad class of materials in which the weight concentration of polymer ranges from 2% to 90%, depending on the application and type of polymer used. Dispersions, wherein the liquid crystal forms nearly spherical droplets randomly distributed throughout a polymer matrix, and the polymer concentration is 20% or more, are normally referred to as polymer dispersed liquid crystals (PDLC). Normally, PDLCs are light scattering in the “off” state and transparent in the “on” state. It is also possible to make reverse mode PDLCs. The display modes, however, cannot be interchanged.
PNLCs are formed by photopolymerization of a mixture containing less than 10% of a reactive monomer in an aligned liquid crystal host, such as a nematic, ferroelectric, or cholesteric phase liquid crystal material. The alignment may be assisted by surface alignment layers or by external fields. The polymerization induces phase separation of an initially homogeneous mixture. The morphology of the polymer network depends on the orientational order of the liquid crystal, properties of the monomer, and the presence of external aligning fields and/or conventional alignment layers applied to the cell surfaces. Normally, PNLCs work as reverse mode PDLCs. It is also possible to make PNLCs that are opaque at zero fields. Once made, however, the display modes cannot be interchanged. The switching times in PDLCs and PNLCs are typically over a millisecond, which is not optimal for most video applications. Moreover, the viewing angle and transmittance of the clear state are limited.
Thus, most liquid crystals switch relatively slowly (over one millisecond). This speed is insufficient for many applications, such as beam steering, spatial light modulators, Deep Fade Protector, Modulated Retro-Reflector, and many others. Such applications require much faster switching times in order for liquid crystals to be effectively used in place of other display systems.
So far the electroclinic effect (electric field-induced director tilt) near the paraelectric—ferroelectric (i.e. a SmA*-SmC*) phase transition offer the fastest liquid crystal switching, however the alignment is unstable due to the layer spacing modulation during switching and has strong temperature dependences, restricting the effect in about 5 degrees range around the SmA*-SmC* phase transition. Ferroelectric liquid crystals (FLCs) also offer fast, about 10-100 microseconds switching without the temperature range restrictions, but the switching ruins the alignment on the long run due to the field induced mechanical deformations associated with the piezoelectric nature of the materials.
In light of the foregoing, it is evident that there is a need in the art for an electro-optical liquid crystal device which has faster switching times, a wider viewing angle, improved stability and low threshold voltage. It would be additionally advantageous if the liquid crystal device contained electro-optical storage functionality.